Automatic electric circuit breakers, particularly those designed for industrial applications, are increasingly being equipped with electronic, i.e., static, trip units in lieu of the traditional thermal and magnetic trip units. These static trip units monitor the phase currents of a distribution circuit and operate to initiate circuit interruption in response to overcurrent conditions ranging from light overload to heavy short circuit proportions. In addition, static trip units can be readily supplemented so as to respond to imbalances in the currents going to and returning from the loads or current flow in a ground return path of the distribution circuit, so as to trip the circuit breaker in response to a ground fault condition.
With such multi-function circuit protective capability of modern static trip units, it is extremely beneficial to know, from a trouble-shooting standpoint, which of the various abnormal circuit conditions precipitated tripping of the circuit breaker. That is, a positive indication that the circuit breaker tripped because of an overload condition, a short-circuit condition, or a ground fault condition can greatly expedite the search for the source of the condition, and thus corrective measures can be taken all the sooner. In industrial applications, a power interruption means the curtailment or complete halt in production, which is extremely costly. Thus, it is highly desirable that a power interruption, when it does occur, be held to as short a duration as possible.
It is accordingly an object of the present invention to provide in a circuit breaker static trip unit indicator means for signaling which of several possible abnormal current conditions precipitated tripping of the circuit breaker.
An additional object is to provide indicator means of the above character which includes a plurality of target indicators which are selectively activated coincident with tripping of the circuit breaker to indicate whether the breaker was tripped in response to an overload condition, a short-circuit condition, or, if so equipped, a ground fault condition.
Yet another object of the present invention is to provide indicator means of the above character, which includes a common target control circuit for the various target indicators.
A further object is to provide a target control circuit of the above character which is completely fail-safe in operation, in that a malfunction in the target circuit or any one of the targets cannot jeopardize tripping of the circuit breaker.
Still another object of the present invention is to provide a target control circuit of the above character which is efficient and reliable in operation, and capable of affording an unambiguous indication of the trip precipitating abnormal current condition.
Other objects of the invention will in part be obvious and in part appear hereinafter.